fix: ship bubblewrap for DSH's command sandbox, keep agents usable without it (#35)

* fix(example): keep agents usable when DSH's inner sandbox has no backend

DSH confines bash commands itself (bubblewrap, else Landlock) and fails closed
with SANDBOX_UNAVAILABLE when no runner is usable. In this hardened pod -
non-root, capabilities dropped, seccomp RuntimeDefault - bubblewrap cannot
start (it needs CAP_SYS_ADMIN) and the pod exposes no Landlock runner, so the
shipped workspace-write default made every command fail:

  sandbox mode "workspace-write" is requested but no sandbox backend is usable

The DSH base bundle at the pinned ref documents DSH_PERMISSION_MODE as the
explicit deployment override, so set it to danger-full-access in both sandbox
templates. The per-user pod is the isolation boundary in this example (its own
pod, PVC and Service; non-root; all capabilities dropped), so a working agent
is worth more than an inner file policy that cannot be enforced. Note that
danger-full-access also turns approval prompts off.

Trade-off documented rather than hidden: an agent command can now read its own
DSH_HOME (including .credentials.yaml) and write anywhere in the container.
Hosts where a runner works should keep workspace-write or read-only.

- main-image docs: DSH_PERMISSION_MODE row plus a note on the runners and the
  fail-closed behavior
- example README: limits bullet and a troubleshooting row with the in-pod
  backend check and how to restore a confined mode
- architecture: tenant-isolation bullet extended
- skill runbook/troubleshooting: operation guidance; never grant CAP_SYS_ADMIN
  or seccompProfile: Unconfined to "fix" this without an explicit decision

Verified: make check (14 manifests), make lint, YAML assertions on both
templates, and the override expression at the pinned dsh-v0.2.1-rc.5 base
bundle (mode: !!js process.env.DSH_PERMISSION_MODE ?? 'workspace-write').

* fix(image): install bubblewrap for DSH's command sandbox

DSH's Linux sandbox runner chain is bubblewrap, then Landlock, and it fails
closed with SANDBOX_UNAVAILABLE when neither works. The image shipped neither,
so a sandboxed agent had no way to execute confined commands.

Add bubblewrap to the runtime stage. Debian ships it unsetuid (0.12.0), so it
still needs unprivileged user namespaces plus the namespace/mount syscalls: in a
hardened pod (all capabilities dropped, RuntimeDefault seccomp) it usually
cannot create one, and DSH then falls back to Landlock or, failing that, to the
DSH_PERMISSION_MODE=danger-full-access override the templates already set.
Where the host allows an unprivileged user namespace, the first-choice runner
now exists, so the confined modes can be restored.

Docs corrected to match: bubblewrap is no longer "absent"; the blocker is the
unsetuid binary's namespace requirement, not a missing CAP_SYS_ADMIN grant.

Verified: make check, make lint; the package exists in the pinned base
(node:24-trixie-slim, bubblewrap 0.12.0-1~deb13u1); a throwaway container with
the pod's flags (--user 1000 --cap-drop ALL --security-opt no-new-privileges)
still cannot create a namespace, which is why the override stays. No local image
build was run.

* ci(image): build and smoke-test the main image on PRs that touch it

The Dockerfile and the pins in versions.env could only be validated by a merge
to main or by running `make image` by hand; neither gives a PR-time signal.

Add ci-image.yaml: on pull requests that change Dockerfile, .dockerignore or
versions.env (and on manual dispatch) it builds linux/amd64 natively with the
pins sourced from versions.env - no registry login, no push - loads the image
locally and smoke-tests it:

- the packaged CLI runs and reports the pinned version
- the container is the non-root node user (1000) and owns /workspace + DSH_HOME
- the bubblewrap command-sandbox runner is present
- the OCI version label matches versions.env
- a container serves /favicon.svg (the same unauthenticated probe the shipped
  manifests use), with logs filtered so the launch token is never printed

Path-filtered like the other on-demand checks, so it is not a required status
check; the published multi-arch build stays in build-main.yaml and release.yaml.

Docs updated in the same change: AGENTS.md repository map and testing table,
docs/gitflow.md checks section, docs/main-image.md build section.

Verified: actionlint 1.7.12 and yamllint 1.38.0 (CI versions) clean over the
whole repository, plus make check and make lint.

---------

Co-authored-by: SREDevOpsAgents <agents@sredevops.org>

AGENTS.md

@@ -49,7 +49,7 @@ before changing anything; it encodes the conventions the CI enforces.
 | examples/minimal-single-tenant/ | The just-one-DSH-Web quickstart |
 | scripts/ | check-versions.sh, check-examples.sh, check-manifests.py, set-version.sh |
 | .dsh/skills/ | Project-local DSH skills (auto-discovered); dsh-agent-sandbox-platform covers platform operation, MCP limits and extension recipes |
-| .github/workflows/ | ci.yaml (lint, dependency-review), ci-python.yaml, ci-manifests.yaml, reusable _build-image.yaml, build-*, release.yaml, e2e-kind.yaml, pr-title.yaml, codeql.yml |
+| .github/workflows/ | ci.yaml (lint, dependency-review), ci-python.yaml, ci-manifests.yaml, ci-image.yaml (amd64 build + smoke test on Dockerfile/versions.env changes), reusable _build-image.yaml, build-*, release.yaml, e2e-kind.yaml, pr-title.yaml, codeql.yml |
 
 ## Commands
 
@@ -74,7 +74,7 @@ needs kind plus the agent-sandbox CRDs (see the example README).
 | Example manifest | make check; kubectl apply --dry-run=server in CI |
 | Optional hardening (gVisor, nodeSelector, NetworkPolicy, provider keys) | make check; server dry-run in CI; manual verification on a capable cluster |
 | Gateway code | make test; make lint; add or update tests |
-| Dockerfile / versions.env | make check; make image (amd64) and a container smoke test |
+| Dockerfile / versions.env | make check; the CI `build-amd64` job (`ci-image.yaml`) builds and smoke-tests the image on the PR; run `make image` locally only when you need the image |
 | Workflow | actionlint in CI; keep actions SHA-pinned; no hard-coded versions |
 | New example | make check; all files described in docs/examples.md |
 | Project skill (.dsh/skills/) | make check; frontmatter and size check; shellcheck skill scripts; load it with the skill tool |